1. Field of the Invention
The invention relates to structural joining of advanced superalloy components during fabrication and/or repair. In some embodiments, the invention relates to surface repair of superalloy turbine blades and vanes in steam or gas turbines, by use of splice inserts that are affixed to a new or repaired substrate by projection resistance heat brazing under contact pressure, in a manner that does not significantly reduce mechanical structural or material properties of the joined components.
2. Description of the Prior Art
“Structural” repair of gas turbine or other superalloy components is commonly recognized as replacing damaged material with matching alloy material and achieving properties, such as strength, that are close to the original manufacture component specifications (e.g., seventy percent ultimate tensile strength of the original specification). For example, it is preferable to perform structural repairs on turbine blades that have experienced surface cracks, so that risk of further cracking is reduced, and the blades are restored to original material structural and dimensional specifications.
Structural repair or new fabrication of nickel and cobalt based superalloy material that is used to manufacture turbine components, such as cast turbine blades, is challenging, due to the metallurgic properties of the finished blade material. For example, as shown in FIG. 1, a superalloy having more than 6% aggregate aluminum or titanium content, such as CM247 alloy, is more susceptible to strain age cracking when subjected to high-temperature welding than a lower aluminum-titanium content X-750 superalloy. The finished turbine blade alloys are typically strengthened during post casting heat treatments which render them difficult to perform subsequent structural welding. Currently used welding processes for superalloy structural fabrication or repair generally involve substantial melting of the substrate adjoining the weld preparation, and complete melting of the welding rod or other filler material added. When a blade constructed of such a material is welded with rods of the same or similar alloy, the blade is susceptible to solidification (aka liquation) cracking within and proximate to the weld, and/or strain age (aka reheat) cracking during subsequent heat treatment processes intended to restore the superalloy original strength and other material properties comparable to a new component.
A past attempt to perform traditional “spot” electric resistance welding of superalloys, in a heat resistance joining apparatus, by passing current between compressed electrodes into a pair of abutting superalloy components also caused solidification cracking within the weld zone. Alternative superalloy welding processes, including laser microcladding with chill fixtures, welding in so-called “hot” boxes at elevated temperatures, and inertia friction welding may still lead to post weld heat treatment strain age cracking. Other friction welding processes, such as friction stir welding, can reduce superalloy cracking propensity, but the employed welding apparatus has relatively limited tool life. The same cracking concerns occur during superalloy component fabrication, when separate components constructed of superalloy material are joined by welding processes.
In comparison to structural repair or fabrication, “cosmetic” repair or fabrication of superalloys is recognized as replacing damaged material (or joining two components of newly fabricated material) with unmatching alloy material of lesser structural property specifications, where the localized original structural performance is not needed. For example, cosmetic repair may be used in order to restore the repaired component's original profile geometry. As noted above, it is desirable to perform structural repairs on surface cracks in order to reduce their likelihood of subsequent spreading when the component is returned to service. Conversely, an example of cosmetic repair is for filling surface pits (as opposed to structural cracks) on a turbine blade airfoil in order to restore its original aerodynamic profile, where the blade's localized exterior surface is not critical for structural integrity of the entire blade. Cosmetic repair or fabrication is often achieved by using oxidation resistant weld or braze alloys of lower strength than the blade body superalloy substrate, but having higher ductility and lower application temperature that does not negatively impact the superalloy substrate's material properties.
In the past, electric resistance brazing has been commonly used for cosmetic metal joining of common ferrous and non-ferrous (e.g., copper) alloys. As shown in FIG. 2, brazing is performed by passing current between compressed electrodes 22, 24 into a pair of abutting components 30, 32. Solid sheet, powder or paste brazing alloy 34 is interposed between the components 30, 32. As electric current is passed between the electrodes 22, 24 the brazing alloy 34 melts and affixes the common ferrous or non-ferrous alloy components 30, 32 to each other. However, the present inventor has no knowledge of past attempts to join superalloy components by a resistance brazing process. Moreover, filling a superalloy component surface crack with braze alloy may not be considered a sufficiently conservative repair approach to avoid future crack growth when the component is returned into service.
Given the shortcomings of superalloy structural repair welding, often the only commercially acceptable solution is to scrap damaged turbine blades that require structural repair, because past experience has shown limited success of such structural repairs. Thus repairs have been limited to those that have in the past been proven to be performed successfully by alternative superalloy welding processes described above, or by cosmetic welding, employing more ductile welding rod filler materials with reduced structural strength.
Thus, a need exists in the art for a method for performing structural joining or repairs on surfaces of superalloy components, such as turbine vanes and blades, so that subcomponents can be joined; or that structural cracks and other surface defects can be repaired.
Another need exists in the art to increase successful rates of structural repairs on surfaces of superalloy components, such as turbine vanes and blades, so that damaged blade scrap rates can be reduced.
Yet another need exists in the art for a method for performing structural joining or repairs on surfaces of superalloy components, such as turbine vanes and blades, with proven, repeatable repair techniques and machinery, that do not require complex welding or post-repair heat treatment procedures.